1. Field of the Invention
The present invention relates to a process and apparatus for composite electroplating a metallic materials. More particularly, the present invention relates to a process and apparatus for composite electroplating a surface of a metallic material with a plating liquid containing metal ions and fine solid particles.
2. Description of the Related Art
In a conventional composite electroplating process, it is well known that a composite electroplating liquid bath is prepared by dispersing fine solid particles in an electroplating liquid containing metal ions, and a metal material is immersed in the composite electroplating liquid bath, while fully stirring the composite electroplating liquid by a propeller-stirring method, pump-circulating method or air-stirring method so as to evenly disperse the fine solid particles in the electroplating liquid. This composite process is disadvantageous for the following reasons.
(1) When the propeller-stirring method is applied, the composite electroplating liquid flows in one direction to form a parallel flow, and thus it is difficult to cause the fine solid particles to be uniformly distributed in the composite electroplating liquid. Where this method is applied to electroplate, for example, an inside peripheral surface of a cylinder, a portion of the composite electroplating liquid brought into contact with the inside peripheral surface of the cylinder flows at a reduced flow speed, and thus the resultant composite plating layer formed on the inside peripheral surface of the cylinder has an uneven surface. Also, it is difficult to uniformly distribute the fine solid particles in the resultant composite electroplating layer.
(2) When the pump-circulating method is applied, the same problems as that of the propeller-stirring method occur.
(3) When the air-stirring method is applied, a plurality of air-blowing pipes (nozzles) are connected to a vessel containing the composite electroplating liquid. In this apparatus, it is difficult to control the air-blowing operation so that air is blown uniformly into the composite electroplating liquid through all the pipes. Therefore, it is difficult to maintain the fine solid particles in a uniformly distributed condition in the electroplating liquid throughout the electroplating operation. When an inside peripheral surface of a metal cylinder is electroplated with the composite electroplating liquid, it is difficult to flow the composite electroplating liquid at a uniform and constant flow rate, while maintaining a uniform distribution of the fine solid particles in the composite electroplating liquid. Therefore, it is difficult to uniformly distribute a desired amount of the fine solid particles in the resultant composite electroplating layer.
Where a composite electroplating is applied to a portion of a metal material surface, for example, an inside peripheral surface of a metal cylinder, Japanese Unexamined Patent Publication (Kokai) No. 52-93,636 teaches a process in which a composite electroplating liquid is introduced into a bottom portion of the cylinder, flows upward through the hollow space of the cylinder and then is overflowed from the top portion of the cylinder. This method is the so-called an upflow method. This method is, however, disadvantageous for the following reasons.
In this upflow method, since an anode is placed in the hollow space of the cylinder, the stream of the composite electroplating liquid flowing through the hollow space of the cylinder in an upward direction is obstructed by the anode, and local differential flow rates are produced in the stream. These local differential flow rates cause the fine solid particles dispersed in the composite electroplating liquid stream to be unevenly distributed. Therefore, in the resultant composite electroplating layer formed on the inside peripheral surface of the cylinder, the fine solid particles are unevenly distributed.
Generally, to form a composite electroplating layer having a uniform distribution of the fine solid particles on a metal material surface by the upflow method, it is required that (1) the flow condition of the composite electroplating liquid is maintained uniform and (2) the fine solid particles dispersed in the composite electroplating liquid are brought in a uniform distribution thereof toward the surface of the metal material to be plated and fixed on the surface.
Nevertheless, in the conventional composite electroplating methods and the conventional productions of composite electroplated metal materials, have not yet succeeded in satisfing the above-mentioned two requirements.